1. Field of the Invention
The present invention relates to a monolithic ceramic capacitor, and more particularly, it relates to an improvement in the structure of a connection between the internal and external electrodes.
2. Description of the Background Art
In general, a monolithic ceramic capacitor comprises a ceramic laminate including a plurality of stacked dielectric ceramic layers, a plurality of internal electrodes which are arranged between adjacent ones of the dielectric ceramic layers, and external electrodes which are formed on both end surfaces of the ceramic laminate and electrically connected with related ones of the internal electrodes.
The internal electrodes have generally been made of a metal which is not molten at a temperature which is appropriate for sintering the dielectric ceramic layers and not oxidized even if the same is fired under such high oxygen partial pressure that the ceramic material is not brought into a semiconductor state. The external electrodes have been formed by applying silver paste, which contains silver powder, for example, onto prescribed surfaces of the fired ceramic laminate having the internal electrodes embedded therein and baking the same.
The external electrodes of such a monolithic ceramic capacitor are typically soldered to a wiring substrate, to electrically connect the monolithic ceramic capacitor with the wiring substrate. However, it has been recognized that when soldered a noble metal, such, as silver, forming the external electrodes is melted and incorporated into the solder in the so-called solder leaching phenomenon. Such solder leaching leads to a reduction in the quality of the monolithic ceramic capacitor. Thus, the external electrodes have generally been covered with plating films of a metal, such as nickel or copper, which is resistant against heat and hardly melts when soldered. Further, an attempt has recently been made to plate such metal plating solder, in order to improve solderability.
As hereinabove described, the plating films of the metal such as nickel or copper formed on the external electrodes are effective for preventing solder leaching, and the plating films of solder are effective for improving solderability. However, the electrical properties of the monolithic ceramic capacitor are reduced by such means. In more concrete terms, metal baking layers formed on both end surfaces of the ceramic laminate for defining the external electrodes. These layers are generally in porous states with fine voids. When such metal baking layers are dipped in a plating solution for forming the metal plating films, the plating solution infiltrates into the voids of the metal baking layers. This plating solution is diffused to reach junction interfaces between the internal electrodes and the dielectric ceramic layers, and reduces junction strength at the junction interfaces while changing dielectric constants, insulation resistance etc. of the dielectric ceramic layers Consequently, the electrical properties of the monolithic ceramic capacitor are reduced.
In the ceramic laminate for the monolithic ceramic capacitor, the junction interfaces between the internal electrodes and the dielectric ceramic layers are exposed to the exterior in a stage before formation of the external electrodes. In such a ceramic laminate, cracks are easily caused along the junction interfaces, which are inferior in mechanical strength to other portions. If bending stress or impact is applied to the ceramic laminate, therefore, cracks may be caused along the junction interfaces which may separate the internal electrodes from the dielectric ceramic layers in the worst case. The aforementioned bending stress and impact are often applied to the ceramic laminate when the step of forming the external electrodes on the ceramic laminate is carried out with an automatic machine. The cracks along the junction interfaces reduce the life of the monolithic ceramic capacitor.